Of the testing devices that are used in coin-operated machines there are several different kinds in which the tests performed on the coins produce results which are dependent on the diameter of the coin under test. In our U.S. Pat. No. 3,870,137 we described an apparatus in which the coin under test rolls on an inclined track along a passageway formed between two closely-spaced plates which are themselves inclined slightly to the vertical so that the coins bear against one wall of the passageway. For each denomination of coin which the apparatus is intended to accept there is an inductor located in the wall of the passageway against which the coins bear. The inductors are circular and correspond in diameter to the coins of their respective denominations. Each is positioned in the wall at such a height above the track as to be coaxial with coins of its respective denominations when they are on the track alongside the inductor. The inductors are connected in oscillating circuits which in the absence of a coin idles at a frequency of say 300 to 400 kHz. When a coin is present alongside one of the inductors the frequency of the oscillating circuit shifts to a value which is dependent on the coin's diameter. By comparing the maximum frequency shift with standard values for acceptable coins of the respective denomination, the coin can be identified as acceptable or unacceptable for that denomination.
The inductive testing apparatus of the kind described above is sensitive to fairly small deviations in diameter from the diameter of the acceptable coin. However, the sensitivity decreases the greater the deviation from the diameter of the acceptable coin so that it becomes difficult to distinguish between two coins of slightly different diameter if the two coins are substantially larger or substantially smaller in diameter than the inductor. It is for this reason that in the apparatus described above a different inductor is used for each size of acceptable coin in order to provide a reliable test of high selectivity.
The manufacturer of coin testing mechanisms may have to supply machines which accept many different sets of coins to meet customers requirements throughout the world. It is a disadvantage in these circumstances to have to match the sizes of the inductors to the sizes of the coins in each coin set.